The manufacture of molded plastic parts made of polymers can be broadly stated as taking place in two distinct steps. The monomeric material used to make the polymer is usually polymerized separately prior to the shaping stage of the plastic part. There are, of course, exceptions; however generally speaking the polymer, once made, is ground into relatively fine particles or pelletized and then heated to liquefy the polymer or alternatively a solvent is added to dissolve the polymer. Subsequently, the liquefied polymer is either injected into a mold, extruded, spun or in some instances blow molding takes place. There are situations where direct casting is performed; however, in casting, and for that matter in injection molding, there exists an unacceptable degree of shrinkage and flow-induced molecular orientation for the most precisely dimensioned parts. When a solvent is used to liquefy a polymer for later solvent devolatilization in an attempt to obtain stress free geometric shapes, concomitant shrinkage caused by solvent evaporation results in cracks and unacceptable stresses in the hardened polymer.
Similarly, if in situ polymerization is attempted, the pre-polymer reaction mixture or raw material that will make up the polymer may shrink upon polymerization up to 20 percent. Thus, neither in situ polymerization or molding of an already formed polymer has proved successful in manufacture of precision stress free plastic parts. To compound the problem, when the molded part exceeds certain thicknesses, cavitation due to shrinkage will frequently leave unacceptable bubbles in the part. These bubbles are indicative of internal stresses.
Three exemplary fields exist where polymers are appropriate for use; however, the limitations set forth above, that is shrinkage of polymers at mold time or at polymerization time, limits usage of polymers for precision articles. Similarly shrinkage of a dissolved polymer upon devolatilization also limits use of parts formed in this method.
When the article is to be used in an optical application, e.g., a large lens, a second requirement, in addition to precise dimensions, is present. That is, there can exist no internal stresses in the article as internal stresses will result in birefringence. Such is the case in injection molding of large plastic lenses and injection molding of optical or magnetic data storage discs currently used in compact disc recorders and personal computers and anticipated as being used as storage media for data in other computer systems. Both these products are now injection molded; however, both with the above restrictions. In the case of the lenses, it is common to utilize a glass blank of a size somewhat smaller, but generally conforming to the lens curvature desired in the final product. In this instance the monomer is polymerized about the glass blank in a relatively thin film. However, blanket exposure of the entire lens leaves bubbles and internal stresses that would occur anywhere but more frequently in narrow spaces. These defects appear often in the center or the thickest part of the lenses if the lenses were made entirely of the plastic material. The present invention addresses such problems in both types of construction.
In the case of the disc, injection molding generally results in some flow-induced and thermoplastic internal stresses. Such stresses cause birefringence and thus the storage capacity and data detection reliability of the disc are limited.
In the third example, a liquefied polymer sheet, when cast onto a flat or curved support will warp or crack if allowed to devolatilize or coagulate over the entire surface simultaneously. Such warping or cracking is caused by shrinkage due to solvent loss.
This invention discloses a method for in situ formation of precisely dimensioned precision parts made of polymers either by in situ polymerization or differential devolatilization or coagulation.
It is an object of this invention to provide a method that permits in situ molding of parts formed of polymers without internal stresses.
It is also an object of this invention to provide a method that permits differential polymerization in a precision mold thereby eliminating voids caused by shrinkage.
It is a further object of this invention to provide a method of casting a polymer sheet by devolatilization or coagulation that avoids surface cracking of the film.
It is also an object of this invention to provide a method that permits in situ polymerization of relatively thick and large precision parts without internal bubbles.
It is still another object of this invention to provide a method that permits casting of precision polymer parts with widely varying thicknesses.
It is a object of this invention to provide an method for differential polymerization of variable thickness precision parts.